## College Physics (4th Edition)

(a) The elevator's upward acceleration is $2.2~m/s^2$ (b) Luke's weight is $784~N$
(a) The apparent weight of the box is $1200~N-960~N = 240~N$ We can find the upward acceleration: $F_N - mg = ma$ $a = \frac{F_N-mg}{m}$ $a = \frac{240~N-(20.0~kg)(9.80~m/s^2)}{20.0~kg}$ $a = 2.2~m/s^2$ The elevator's upward acceleration is $2.2~m/s^2$ (b) We can find Luke's mass: $F_N - mg = ma$ $F_N = m~(g+a)$ $m = \frac{F_N}{g+a}$ $m = \frac{960~N}{9.80~m/s^2+2.2~m/s^2}$ $m = 80~kg$ Luke's weight is $(80~kg)(9.80~m/s^2) = 784~N$